Seoul, South Korea (CNN) — North Korea has stepped up activity at its main nuclear test site, possibly preparing to carry out a fourth underground blast, South Korea said Tuesday.

The report comes just days before U.S. President Barack Obama is due to arrive in South Korea as part of a visit to several Asian countries.

“We confirm that we have spotted several activities related to the nuclear test in Punggye-ri in North Korea,” the South Korean Defense Ministry said in a statement. It declined to specify what the activities were, saying the information was classified.

The ministry said it had increased its military preparedness since Monday morning and was monitoring around the clock for signs of activity at the Punggye-ri site in a northeastern region of North Korea.

North Korea said last month that it wouldn’t rule “a new form of a nuclear test” to strengthen its nuclear deterrent. Experts have speculated that that could refer to the testing of a uranium bomb.

3 previous tests

The reclusive regime in Pyongyang is known to have conducted three previous tests, all of them believed to be based on plutonium. The most recent one took place February 2013.

The past tests prompted international condemnation and the imposition of sanctions aimed at hampering the North’s weapons program.

Since the last underground detonation, South Korean officials have repeatedly said that they believe North Korea is ready to carry out another test at any time, pending a political decision to go ahead with it.

“A lot of activity is currently being seen, so our forces are keeping in mind the possibility that North Korea may suddenly conduct a nuclear test in a short period of time, or as in previous cases, deceive us with what appears to be a nuclear test,” Kim Min-seok, a spokesman for the South Korean Defense Ministry said Tuesday, according to the local news agency Yonhap.

Obama visit ‘dangerous’

The reports of activity come ahead of Obama’s South Korean visit, which begins Friday.

The North Korean Foreign Ministry on Monday described Obama’s trip as “a reactionary and dangerous one as it is aimed to escalate confrontation and bring dark clouds of a nuclear arms race to hang over this unstable region.”

North Korean Leader Kim Jong Un’s regime carried out a series of missile and rocket launches in March, prompting condemnation from the U.N. Security Council.

An artillery exercise on March 31 near the border with South Korea resulted in a brief exchange of fire between the two sides after North Korean ordnance landed in South Korean waters.

North Korea’s series of provocative acts were seen as a response to joint U.S.-South Korean military drills in the region. The exercises take place each spring and regularly draw angry reactions from Pyongyang.

CNN’s Jethro Mullen reported and wrote from Hong Kong, and journalist Stella Kim reported from Seoul.